Vacuum tubes ("vacutainers") are presently the most commonly-used sample tubes. A blood sample is usually taken by means of a cannula pointed at both ends, one tip being inserted into a patient's vein whereas the other tip is inserted into an evacuated vacuum tube through the stopper thereof. When the connection has been made, the difference in pressure causes the blood sample to flow into the vacuum tube.
After the sample has been taken, the vacuum tube usually remains at negative pressure. If the stopper is removed in the laboratory in order to take the sample out of the vacuum tube, the sudden equalization in pressure may result in an air stream entering the tube, so that fragments of sample are sprayed out. There will thus be a serious risk of infection, because the samples (e.g. blood samples) in vacuum tubes are normally considered as possibly infectious. Laboratory staff handling these samples run the risk of being infected by any disease transmitted by a sample. It is therefore very important to reduce the handling of samples to a minimum and to eliminate dangerous manipulations.
This problem was addressed in the prior art by means of a pipetting insert (PCT patent application, publication number WO 87/05208) which contains a pipetting duct ending in a tip for piercing the stopper of the vacuum tube when the pipetting insert is placed thereon. The pipetting duct has two functions. On the one hand it equalizes the pressure, so that after the pipetting insert has been fitted, the pressure in the tube is equal to the air pressure. On the other hand the pipetting duct serves as a guide element for the suction tube or needle of a pipetting device for taking the sample out of the tube.
However, when the pipetting insert known in the art is inserted through the stopper of the vacuum tube there is a sudden equalization of pressure, so that if the pressure difference is relatively large, fragments of sample are sprayed out of the tube, with the aforementioned risk of infection.
Additionally, as soon as the known pipetting insert has been inserted through the stopper of the vacuum tube, the pipetting duct connects the interior of the tube to the environment, i.e. a sample-containing tube is open. There is therefore a risk that when the tube is handled, fragments of sample will escape from the tube and cause a risk of infection. Also, a part of the sample may be lost by evaporation.